Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].¶

Conformance criteria and considerations regarding error handling are defined in Section 2.5.¶

This specification uses the Augmented Backus-Naur Form (ABNF) notation of [RFC5234] with the list rule extension defined in Appendix B. Appendix C shows the collected ABNF with the list rule expanded.¶

The following core rules are included by reference, as defined in [RFC5234], Appendix B.1: ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote), HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF (line feed), OCTET (any 8-bit sequence of data), SP (space), and VCHAR (any visible [USASCII] character).¶

As a convention, ABNF rule names prefixed with "obs-" denote "obsolete" grammar rules that appear for historical reasons.¶

HTTP is a stateless request/response protocol that operates by exchanging messages (Section 3) across a reliable transport or session-layer "connection" (Section 6). An HTTP "client" is a program that establishes a connection to a server for the purpose of sending one or more HTTP requests. An HTTP "server" is a program that accepts connections in order to service HTTP requests by sending HTTP responses.¶

The terms client and server refer only to the roles that these programs perform for a particular connection. The same program might act as a client on some connections and a server on others. We use the term "user agent" to refer to any of the various client programs that initiate a request, including (but not limited to) browsers, spiders (web-based robots), command-line tools, native applications, and mobile apps. The term "origin server" is used to refer to the program that can originate authoritative responses to a request. For general requirements, we use the terms "sender" and "recipient" to refer to any component that sends or receives, respectively, a given message.¶

HTTP relies upon the Uniform Resource Identifier (URI) standard [RFC3986] to indicate the target resource (Section 5.1) and relationships between resources. Messages are passed in a format similar to that used by Internet mail [RFC5322] and the Multipurpose Internet Mail Extensions (MIME) [RFC2045] (see Appendix A of [Part2] for the differences between HTTP and MIME messages).¶

Most HTTP communication consists of a retrieval request (GET) for a representation of some resource identified by a URI. In the simplest case, this might be accomplished via a single bidirectional connection (===) between the user agent (UA) and the origin server (O).¶

A client sends an HTTP request to a server in the form of a request message, beginning with a request-line that includes a method, URI, and protocol version (Section 3.1.1), followed by header fields containing request modifiers, client information, and representation metadata (Section 3.2), an empty line to indicate the end of the header section, and finally a message body containing the payload body (if any, Section 3.3).¶

A server responds to a client's request by sending one or more HTTP response messages, each beginning with a status line that includes the protocol version, a success or error code, and textual reason phrase (Section 3.1.2), possibly followed by header fields containing server information, resource metadata, and representation metadata (Section 3.2), an empty line to indicate the end of the header section, and finally a message body containing the payload body (if any, Section 3.3).¶

A connection might be used for multiple request/response exchanges, as defined in Section 6.3.¶

client request:

GET /hello.txt HTTP/1.1
User-Agent: curl/7.16.3 libcurl/7.16.3 OpenSSL/0.9.7l zlib/1.2.3
Host: www.example.com
Accept-Language: en, mi

server response:

HTTP/1.1 200 OK
Date: Mon, 27 Jul 2009 12:28:53 GMT
Server: Apache
Last-Modified: Wed, 22 Jul 2009 19:15:56 GMT
ETag: "34aa387-d-1568eb00"
Accept-Ranges: bytes
Content-Length: 14
Vary: Accept-Encoding
Content-Type: text/plain

Hello World!

(Note that the content length includes the trailing CR/LF sequence of the body text)

When considering the design of HTTP, it is easy to fall into a trap of thinking that all user agents are general-purpose browsers and all origin servers are large public websites. That is not the case in practice. Common HTTP user agents include household appliances, stereos, scales, firmware update scripts, command-line programs, mobile apps, and communication devices in a multitude of shapes and sizes. Likewise, common HTTP origin servers include home automation units, configurable networking components, office machines, autonomous robots, news feeds, traffic cameras, ad selectors, and video delivery platforms.¶

The term "user agent" does not imply that there is a human user directly interacting with the software agent at the time of a request. In many cases, a user agent is installed or configured to run in the background and save its results for later inspection (or save only a subset of those results that might be interesting or erroneous). Spiders, for example, are typically given a start URI and configured to follow certain behavior while crawling the Web as a hypertext graph.¶

The implementation diversity of HTTP means that we cannot assume the user agent can make interactive suggestions to a user or provide adequate warning for security or privacy options. In the few cases where this specification requires reporting of errors to the user, it is acceptable for such reporting to only be observable in an error console or log file. Likewise, requirements that an automated action be confirmed by the user before proceeding can be met via advance configuration choices, run-time options, or simply not proceeding with the unsafe action.¶

HTTP enables the use of intermediaries to satisfy requests through a chain of connections. There are three common forms of HTTP intermediary: proxy, gateway, and tunnel. In some cases, a single intermediary might act as an origin server, proxy, gateway, or tunnel, switching behavior based on the nature of each request.¶

The figure above shows three intermediaries (A, B, and C) between the user agent and origin server. A request or response message that travels the whole chain will pass through four separate connections. Some HTTP communication options might apply only to the connection with the nearest, non-tunnel neighbor, only to the end-points of the chain, or to all connections along the chain. Although the diagram is linear, each participant might be engaged in multiple, simultaneous communications. For example, B might be receiving requests from many clients other than A, and/or forwarding requests to servers other than C, at the same time that it is handling A's request.¶

We use the terms "upstream" and "downstream" to describe various requirements in relation to the directional flow of a message: all messages flow from upstream to downstream. Likewise, we use the terms inbound and outbound to refer to directions in relation to the request path: "inbound" means toward the origin server and "outbound" means toward the user agent.¶

A "proxy" is a message forwarding agent that is selected by the client, usually via local configuration rules, to receive requests for some type(s) of absolute URI and attempt to satisfy those requests via translation through the HTTP interface. Some translations are minimal, such as for proxy requests for "http" URIs, whereas other requests might require translation to and from entirely different application-level protocols. Proxies are often used to group an organization's HTTP requests through a common intermediary for the sake of security, annotation services, or shared caching.¶

An HTTP-to-HTTP proxy is called a "transforming proxy" if it is designed or configured to modify request or response messages in a semantically meaningful way (i.e., modifications, beyond those required by normal HTTP processing, that change the message in a way that would be significant to the original sender or potentially significant to downstream recipients). For example, a transforming proxy might be acting as a shared annotation server (modifying responses to include references to a local annotation database), a malware filter, a format transcoder, or an intranet-to-Internet privacy filter. Such transformations are presumed to be desired by the client (or client organization) that selected the proxy and are beyond the scope of this specification. However, when a proxy is not intended to transform a given message, we use the term "non-transforming proxy" to target requirements that preserve HTTP message semantics. See Section 6.3.4 of [Part2] and Section 7.5 of [Part6] for status and warning codes related to transformations.¶

A "gateway" (a.k.a., "reverse proxy") is a receiving agent that acts as a layer above some other server(s) and translates the received requests to the underlying server's protocol. Gateways are often used to encapsulate legacy or untrusted information services, to improve server performance through "accelerator" caching, and to enable partitioning or load-balancing of HTTP services across multiple machines.¶

A gateway behaves as an origin server on its outbound connection and as a user agent on its inbound connection. All HTTP requirements applicable to an origin server also apply to the outbound communication of a gateway. A gateway communicates with inbound servers using any protocol that it desires, including private extensions to HTTP that are outside the scope of this specification. However, an HTTP-to-HTTP gateway that wishes to interoperate with third-party HTTP servers MUST conform to HTTP user agent requirements on the gateway's inbound connection and MUST implement the Connection (Section 6.1) and Via (Section 5.7.1) header fields for both connections.¶

A "tunnel" acts as a blind relay between two connections without changing the messages. Once active, a tunnel is not considered a party to the HTTP communication, though the tunnel might have been initiated by an HTTP request. A tunnel ceases to exist when both ends of the relayed connection are closed. Tunnels are used to extend a virtual connection through an intermediary, such as when Transport Layer Security (TLS, [RFC5246]) is used to establish confidential communication through a shared firewall proxy.¶

The above categories for intermediary only consider those acting as participants in the HTTP communication. There are also intermediaries that can act on lower layers of the network protocol stack, filtering or redirecting HTTP traffic without the knowledge or permission of message senders. Network intermediaries often introduce security flaws or interoperability problems by violating HTTP semantics. For example, an "interception proxy" [RFC3040] (also commonly known as a "transparent proxy" [RFC1919] or "captive portal") differs from an HTTP proxy because it is not selected by the client. Instead, an interception proxy filters or redirects outgoing TCP port 80 packets (and occasionally other common port traffic). Interception proxies are commonly found on public network access points, as a means of enforcing account subscription prior to allowing use of non-local Internet services, and within corporate firewalls to enforce network usage policies. They are indistinguishable from a man-in-the-middle attack.¶

HTTP is defined as a stateless protocol, meaning that each request message can be understood in isolation. Many implementations depend on HTTP's stateless design in order to reuse proxied connections or dynamically load-balance requests across multiple servers. Hence, servers MUST NOT assume that two requests on the same connection are from the same user agent unless the connection is secured and specific to that agent. Some non-standard HTTP extensions (e.g., [RFC4559]) have been known to violate this requirement, resulting in security and interoperability problems.¶

A "cache" is a local store of previous response messages and the subsystem that controls its message storage, retrieval, and deletion. A cache stores cacheable responses in order to reduce the response time and network bandwidth consumption on future, equivalent requests. Any client or server MAY employ a cache, though a cache cannot be used by a server while it is acting as a tunnel.¶

The effect of a cache is that the request/response chain is shortened if one of the participants along the chain has a cached response applicable to that request. The following illustrates the resulting chain if B has a cached copy of an earlier response from O (via C) for a request that has not been cached by UA or A.¶

A response is "cacheable" if a cache is allowed to store a copy of the response message for use in answering subsequent requests. Even when a response is cacheable, there might be additional constraints placed by the client or by the origin server on when that cached response can be used for a particular request. HTTP requirements for cache behavior and cacheable responses are defined in Section 2 of [Part6].¶

There are a wide variety of architectures and configurations of caches deployed across the World Wide Web and inside large organizations. These include national hierarchies of proxy caches to save transoceanic bandwidth, collaborative systems that broadcast or multicast cache entries, archives of pre-fetched cache entries for use in off-line or high-latency environments, and so on.¶

This specification targets conformance criteria according to the role of a participant in HTTP communication. Hence, HTTP requirements are placed on senders, recipients, clients, servers, user agents, intermediaries, origin servers, proxies, gateways, or caches, depending on what behavior is being constrained by the requirement. Additional (social) requirements are placed on implementations, resource owners, and protocol element registrations when they apply beyond the scope of a single communication.¶

The verb "generate" is used instead of "send" where a requirement differentiates between creating a protocol element and merely forwarding a received element downstream.¶

An implementation is considered conformant if it complies with all of the requirements associated with the roles it partakes in HTTP. Note that SHOULD-level requirements are relevant here, unless one of the documented exceptions is applicable.¶

Conformance applies to both the syntax and semantics of HTTP protocol elements. A sender MUST NOT generate protocol elements that convey a meaning that is known by that sender to be false. A sender MUST NOT generate protocol elements that do not match the grammar defined by the ABNF rules for those protocol elements that are applicable to the sender's role. If a received protocol element is processed, the recipient MUST be able to parse any value that would match the ABNF rules for that protocol element, excluding only those rules not applicable to the recipient's role.¶

Unless noted otherwise, a recipient MAY attempt to recover a usable protocol element from an invalid construct. HTTP does not define specific error handling mechanisms except when they have a direct impact on security, since different applications of the protocol require different error handling strategies. For example, a Web browser might wish to transparently recover from a response where the Location header field doesn't parse according to the ABNF, whereas a systems control client might consider any form of error recovery to be dangerous.¶

HTTP uses a "<major>.<minor>" numbering scheme to indicate versions of the protocol. This specification defines version "1.1". The protocol version as a whole indicates the sender's conformance with the set of requirements laid out in that version's corresponding specification of HTTP.¶

The version of an HTTP message is indicated by an HTTP-version field in the first line of the message. HTTP-version is case-sensitive.¶

The HTTP version number consists of two decimal digits separated by a "." (period or decimal point). The first digit ("major version") indicates the HTTP messaging syntax, whereas the second digit ("minor version") indicates the highest minor version to which the sender is conformant and able to understand for future communication. The minor version advertises the sender's communication capabilities even when the sender is only using a backwards-compatible subset of the protocol, thereby letting the recipient know that more advanced features can be used in response (by servers) or in future requests (by clients).¶

When an HTTP/1.1 message is sent to an HTTP/1.0 recipient [RFC1945] or a recipient whose version is unknown, the HTTP/1.1 message is constructed such that it can be interpreted as a valid HTTP/1.0 message if all of the newer features are ignored. This specification places recipient-version requirements on some new features so that a conformant sender will only use compatible features until it has determined, through configuration or the receipt of a message, that the recipient supports HTTP/1.1.¶

The interpretation of a header field does not change between minor versions of the same major HTTP version, though the default behavior of a recipient in the absence of such a field can change. Unless specified otherwise, header fields defined in HTTP/1.1 are defined for all versions of HTTP/1.x. In particular, the Host and Connection header fields ought to be implemented by all HTTP/1.x implementations whether or not they advertise conformance with HTTP/1.1.¶

New header fields can be defined such that, when they are understood by a recipient, they might override or enhance the interpretation of previously defined header fields. When an implementation receives an unrecognized header field, the recipient MUST ignore that header field for local processing regardless of the message's HTTP version. An unrecognized header field received by a proxy MUST be forwarded downstream unless the header field's field-name is listed in the message's Connection header field (see Section 6.1). These requirements allow HTTP's functionality to be enhanced without requiring prior update of deployed intermediaries.¶

Intermediaries that process HTTP messages (i.e., all intermediaries other than those acting as tunnels) MUST send their own HTTP-version in forwarded messages. In other words, they MUST NOT blindly forward the first line of an HTTP message without ensuring that the protocol version in that message matches a version to which that intermediary is conformant for both the receiving and sending of messages. Forwarding an HTTP message without rewriting the HTTP-version might result in communication errors when downstream recipients use the message sender's version to determine what features are safe to use for later communication with that sender.¶

An HTTP client SHOULD send a request version equal to the highest version to which the client is conformant and whose major version is no higher than the highest version supported by the server, if this is known. An HTTP client MUST NOT send a version to which it is not conformant.¶

An HTTP client MAY send a lower request version if it is known that the server incorrectly implements the HTTP specification, but only after the client has attempted at least one normal request and determined from the response status or header fields (e.g., Server) that the server improperly handles higher request versions.¶

An HTTP server SHOULD send a response version equal to the highest version to which the server is conformant and whose major version is less than or equal to the one received in the request. An HTTP server MUST NOT send a version to which it is not conformant. A server MAY send a 505 (HTTP Version Not Supported) response if it cannot send a response using the major version used in the client's request.¶

An HTTP server MAY send an HTTP/1.0 response to an HTTP/1.0 request if it is known or suspected that the client incorrectly implements the HTTP specification and is incapable of correctly processing later version responses, such as when a client fails to parse the version number correctly or when an intermediary is known to blindly forward the HTTP-version even when it doesn't conform to the given minor version of the protocol. Such protocol downgrades SHOULD NOT be performed unless triggered by specific client attributes, such as when one or more of the request header fields (e.g., User-Agent) uniquely match the values sent by a client known to be in error.¶

The intention of HTTP's versioning design is that the major number will only be incremented if an incompatible message syntax is introduced, and that the minor number will only be incremented when changes made to the protocol have the effect of adding to the message semantics or implying additional capabilities of the sender. However, the minor version was not incremented for the changes introduced between [RFC2068] and [RFC2616], and this revision has specifically avoiding any such changes to the protocol.¶

Uniform Resource Identifiers (URIs) [RFC3986] are used throughout HTTP as the means for identifying resources (Section 2 of [Part2]). URI references are used to target requests, indicate redirects, and define relationships.¶

This specification adopts the definitions of "URI-reference", "absolute-URI", "relative-part", "port", "host", "path-abempty", "query", "segment", and "authority" from the URI generic syntax. In addition, we define an "absolute-path" rule (that differs from RFC 3986's "path-absolute" in that it allows a leading "//") and a "partial-URI" rule for protocol elements that allow a relative URI but not a fragment.¶

Each protocol element in HTTP that allows a URI reference will indicate in its ABNF production whether the element allows any form of reference (URI-reference), only a URI in absolute form (absolute-URI), only the path and optional query components, or some combination of the above. Unless otherwise indicated, URI references are parsed relative to the effective request URI (Section 5.5).¶

An HTTP message can either be a request from client to server or a response from server to client. Syntactically, the two types of message differ only in the start-line, which is either a request-line (for requests) or a status-line (for responses), and in the algorithm for determining the length of the message body (Section 3.3).¶

In theory, a client could receive requests and a server could receive responses, distinguishing them by their different start-line formats, but in practice servers are implemented to only expect a request (a response is interpreted as an unknown or invalid request method) and clients are implemented to only expect a response.¶

A sender MUST NOT send whitespace between the start-line and the first header field. The presence of such whitespace in a request might be an attempt to trick a server into ignoring that field or processing the line after it as a new request, either of which might result in a security vulnerability if other implementations within the request chain interpret the same message differently. Likewise, the presence of such whitespace in a response might be ignored by some clients or cause others to cease parsing.¶

A recipient that receives whitespace between the start-line and the first header field MUST either reject the message as invalid or consume each whitespace-preceded line without further processing of it (i.e., ignore the entire line, along with any subsequent lines preceded by whitespace, until a properly formed header field is received or the header block is terminated).¶

Each HTTP header field consists of a case-insensitive field name followed by a colon (":"), optional whitespace, and the field value.¶

The field-name token labels the corresponding field-value as having the semantics defined by that header field. For example, the Date header field is defined in Section 7.1.1.2 of [Part2] as containing the origination timestamp for the message in which it appears.¶

The message body (if any) of an HTTP message is used to carry the payload body of that request or response. The message body is identical to the payload body unless a transfer coding has been applied, as described in Section 3.3.1.¶

The rules for when a message body is allowed in a message differ for requests and responses.¶

The presence of a message body in a request is signaled by a Content-Length or Transfer-Encoding header field. Request message framing is independent of method semantics, even if the method does not define any use for a message body.¶

The presence of a message body in a response depends on both the request method to which it is responding and the response status code (Section 3.1.2). Responses to the HEAD request method never include a message body because the associated response header fields (e.g., Transfer-Encoding, Content-Length, etc.), if present, indicate only what their values would have been if the request method had been GET (Section 4.3.2 of [Part2]). 2xx (Successful) responses to CONNECT switch to tunnel mode instead of having a message body (Section 4.3.6 of [Part2]). All 1xx (Informational), 204 (No Content), and 304 (Not Modified) responses do not include a message body. All other responses do include a message body, although the body might be of zero length.¶

A server that receives an incomplete request message, usually due to a canceled request or a triggered time-out exception, MAY send an error response prior to closing the connection.¶

A client that receives an incomplete response message, which can occur when a connection is closed prematurely or when decoding a supposedly chunked transfer coding fails, MUST record the message as incomplete. Cache requirements for incomplete responses are defined in Section 3 of [Part6].¶

If a response terminates in the middle of the header block (before the empty line is received) and the status code might rely on header fields to convey the full meaning of the response, then the client cannot assume that meaning has been conveyed; the client might need to repeat the request in order to determine what action to take next.¶

A message body that uses the chunked transfer coding is incomplete if the zero-sized chunk that terminates the encoding has not been received. A message that uses a valid Content-Length is incomplete if the size of the message body received (in octets) is less than the value given by Content-Length. A response that has neither chunked transfer coding nor Content-Length is terminated by closure of the connection, and thus is considered complete regardless of the number of message body octets received, provided that the header block was received intact.¶

Older HTTP/1.0 user agent implementations might send an extra CRLF after a POST request as a lame workaround for some early server applications that failed to read message body content that was not terminated by a line-ending. An HTTP/1.1 user agent MUST NOT preface or follow a request with an extra CRLF. If terminating the request message body with a line-ending is desired, then the user agent MUST count the terminating CRLF octets as part of the message body length.¶

In the interest of robustness, servers SHOULD ignore at least one empty line received where a request-line is expected. In other words, if a server is reading the protocol stream at the beginning of a message and receives a CRLF first, the server SHOULD ignore the CRLF.¶

Although the line terminator for the start-line and header fields is the sequence CRLF, recipients MAY recognize a single LF as a line terminator and ignore any preceding CR.¶

Although the request-line and status-line grammar rules require that each of the component elements be separated by a single SP octet, recipients MAY instead parse on whitespace-delimited word boundaries and, aside from the CRLF terminator, treat any form of whitespace as the SP separator while ignoring preceding or trailing whitespace; such whitespace includes one or more of the following octets: SP, HTAB, VT (%x0B), FF (%x0C), or bare CR.¶

When a server listening only for HTTP request messages, or processing what appears from the start-line to be an HTTP request message, receives a sequence of octets that does not match the HTTP-message grammar aside from the robustness exceptions listed above, the server SHOULD respond with a 400 (Bad Request) response.¶

The chunked transfer coding modifies the body of a message in order to transfer it as a series of chunks, each with its own size indicator, followed by an OPTIONAL trailer containing header fields. This allows dynamically generated content to be transferred along with the information necessary for the recipient to verify that it has received the full message.¶

Chunk extensions within the chunked transfer coding are deprecated. Senders SHOULD NOT send chunk-ext. Definition of new chunk extensions is discouraged.¶

The chunk-size field is a string of hex digits indicating the size of the chunk-data in octets. The chunked transfer coding is complete when a chunk with a chunk-size of zero is received, possibly followed by a trailer, and finally terminated by an empty line.¶

The codings defined below can be used to compress the payload of a message.¶

The "TE" header field in a request indicates what transfer codings, besides chunked, the client is willing to accept in response, and whether or not the client is willing to accept trailer fields in a chunked transfer coding.¶

The TE field-value consists of a comma-separated list of transfer coding names, each allowing for optional parameters (as described in Section 4), and/or the keyword "trailers". Clients MUST NOT send the chunked transfer coding name in TE; chunked is always acceptable for HTTP/1.1 recipients.¶

Three examples of TE use are below.¶

The presence of the keyword "trailers" indicates that the client is willing to accept trailer fields in a chunked transfer coding, as defined in Section 4.1, on behalf of itself and any downstream clients. For chained requests, this implies that either: (a) all downstream clients are willing to accept trailer fields in the forwarded response; or, (b) the client will attempt to buffer the response on behalf of downstream recipients. Note that HTTP/1.1 does not define any means to limit the size of a chunked response such that a client can be assured of buffering the entire response.¶

When multiple transfer codings are acceptable, the client MAY rank the codings by preference using a case-insensitive "q" parameter (similar to the qvalues used in content negotiation fields, Section 5.3.1 of [Part2]). The rank value is a real number in the range 0 through 1, where 0.001 is the least preferred and 1 is the most preferred; a value of 0 means "not acceptable".¶

If the TE field-value is empty or if no TE field is present, the only acceptable transfer coding is chunked. A message with no transfer coding is always acceptable.¶

Since the TE header field only applies to the immediate connection, a sender of TE MUST also send a "TE" connection option within the Connection header field (Section 6.1) in order to prevent the TE field from being forwarded by intermediaries that do not support its semantics.¶

HTTP is used in a wide variety of applications, ranging from general-purpose computers to home appliances. In some cases, communication options are hard-coded in a client's configuration. However, most HTTP clients rely on the same resource identification mechanism and configuration techniques as general-purpose Web browsers.¶

HTTP communication is initiated by a user agent for some purpose. The purpose is a combination of request semantics, which are defined in [Part2], and a target resource upon which to apply those semantics. A URI reference (Section 2.7) is typically used as an identifier for the "target resource", which a user agent would resolve to its absolute form in order to obtain the "target URI". The target URI excludes the reference's fragment identifier component, if any, since fragment identifiers are reserved for client-side processing ([RFC3986], Section 3.5).¶

Once the target URI is determined, a client needs to decide whether a network request is necessary to accomplish the desired semantics and, if so, where that request is to be directed.¶

If the client has a response cache and the request semantics can be satisfied by a cache ([Part6]), then the request is usually directed to the cache first.¶

If the request is not satisfied by a cache, then a typical client will check its configuration to determine whether a proxy is to be used to satisfy the request. Proxy configuration is implementation-dependent, but is often based on URI prefix matching, selective authority matching, or both, and the proxy itself is usually identified by an "http" or "https" URI. If a proxy is applicable, the client connects inbound by establishing (or reusing) a connection to that proxy.¶

If no proxy is applicable, a typical client will invoke a handler routine, usually specific to the target URI's scheme, to connect directly to an authority for the target resource. How that is accomplished is dependent on the target URI scheme and defined by its associated specification, similar to how this specification defines origin server access for resolution of the "http" (Section 2.7.1) and "https" (Section 2.7.2) schemes.¶

HTTP requirements regarding connection management are defined in Section 6.¶

Once an inbound connection is obtained, the client sends an HTTP request message (Section 3) with a request-target derived from the target URI. There are four distinct formats for the request-target, depending on both the method being requested and whether the request is to a proxy.¶

origin-form ¶

The most common form of request-target is the origin-form. When making a request directly to an origin server, other than a CONNECT or server-wide OPTIONS request (as detailed below), a client MUST send only the absolute path and query components of the target URI as the request-target. If the target URI's path component is empty, then the client MUST send "/" as the path within the origin-form of request-target. A Host header field is also sent, as defined in Section 5.4, containing the target URI's authority component (excluding any userinfo).¶

For example, a client wishing to retrieve a representation of the resource identified as¶

followed by the remainder of the request message.¶

absolute-form ¶

When making a request to a proxy, other than a CONNECT or server-wide OPTIONS request (as detailed below), a client MUST send the target URI in absolute-form as the request-target. The proxy is requested to either service that request from a valid cache, if possible, or make the same request on the client's behalf to either the next inbound proxy server or directly to the origin server indicated by the request-target. Requirements on such "forwarding" of messages are defined in Section 5.7.¶

To allow for transition to the absolute-form for all requests in some future version of HTTP, HTTP/1.1 servers MUST accept the absolute-form in requests, even though HTTP/1.1 clients will only send them in requests to proxies.¶

authority-form ¶

The authority-form of request-target is only used for CONNECT requests (Section 4.3.6 of [Part2]). When making a CONNECT request to establish a tunnel through one or more proxies, a client MUST send only the target URI's authority component (excluding any userinfo) as the request-target. For example,¶

asterisk-form ¶

The asterisk-form of request-target is only used for a server-wide OPTIONS request (Section 4.3.7 of [Part2]). When a client wishes to request OPTIONS for the server as a whole, as opposed to a specific named resource of that server, the client MUST send only "*" (%x2A) as the request-target. For example,¶

If a proxy receives an OPTIONS request with an absolute-form of request-target in which the URI has an empty path and no query component, then the last proxy on the request chain MUST send a request-target of "*" when it forwards the request to the indicated origin server.¶

For example, the request

OPTIONS http://www.example.org:8001 HTTP/1.1

would be forwarded by the final proxy as

OPTIONS * HTTP/1.1
Host: www.example.org:8001

after connecting to port 8001 of host "www.example.org".

The "Host" header field in a request provides the host and port information from the target URI, enabling the origin server to distinguish among resources while servicing requests for multiple host names on a single IP address. Since the Host field-value is critical information for handling a request, it SHOULD be sent as the first header field following the request-line.¶

A client MUST send a Host header field in all HTTP/1.1 request messages. If the target URI includes an authority component, then the Host field-value MUST be identical to that authority component after excluding any userinfo (Section 2.7.1). If the authority component is missing or undefined for the target URI, then the Host header field MUST be sent with an empty field-value.¶

The Host header field MUST be sent in an HTTP/1.1 request even if the request-target is in the absolute-form, since this allows the Host information to be forwarded through ancient HTTP/1.0 proxies that might not have implemented Host.¶

When a proxy receives a request with an absolute-form of request-target, the proxy MUST ignore the received Host header field (if any) and instead replace it with the host information of the request-target. If the proxy forwards the request, it MUST generate a new Host field-value based on the received request-target rather than forward the received Host field-value.¶

Since the Host header field acts as an application-level routing mechanism, it is a frequent target for malware seeking to poison a shared cache or redirect a request to an unintended server. An interception proxy is particularly vulnerable if it relies on the Host field-value for redirecting requests to internal servers, or for use as a cache key in a shared cache, without first verifying that the intercepted connection is targeting a valid IP address for that host.¶

A server MUST respond with a 400 (Bad Request) status code to any HTTP/1.1 request message that lacks a Host header field and to any request message that contains more than one Host header field or a Host header field with an invalid field-value.¶

A server that receives an HTTP request message MUST reconstruct the user agent's original target URI, based on the pieces of information learned from the request-target, Host header field, and connection context, in order to identify the intended target resource and properly service the request. The URI derived from this reconstruction process is referred to as the "effective request URI".¶

For a user agent, the effective request URI is the target URI.¶

If the request-target is in absolute-form, then the effective request URI is the same as the request-target. Otherwise, the effective request URI is constructed as follows.¶

If the request is received over a TLS-secured TCP connection, then the effective request URI's scheme is "https"; otherwise, the scheme is "http".¶

If the request-target is in authority-form, then the effective request URI's authority component is the same as the request-target. Otherwise, if a Host header field is supplied with a non-empty field-value, then the authority component is the same as the Host field-value. Otherwise, the authority component is the concatenation of the default host name configured for the server, a colon (":"), and the connection's incoming TCP port number in decimal form.¶

If the request-target is in authority-form or asterisk-form, then the effective request URI's combined path and query component is empty. Otherwise, the combined path and query component is the same as the request-target.¶

The components of the effective request URI, once determined as above, can be combined into absolute-URI form by concatenating the scheme, "://", authority, and combined path and query component.¶

Example 1: the following message received over an insecure TCP connection

GET /pub/WWW/TheProject.html HTTP/1.1
Host: www.example.org:8080

has an effective request URI of

http://www.example.org:8080/pub/WWW/TheProject.html

Example 2: the following message received over a TLS-secured TCP connection

OPTIONS * HTTP/1.1
Host: www.example.org

has an effective request URI of

https://www.example.org

An origin server that does not allow resources to differ by requested host MAY ignore the Host field-value and instead replace it with a configured server name when constructing the effective request URI.¶

Recipients of an HTTP/1.0 request that lacks a Host header field MAY attempt to use heuristics (e.g., examination of the URI path for something unique to a particular host) in order to guess the effective request URI's authority component.¶

HTTP does not include a request identifier for associating a given request message with its corresponding one or more response messages. Hence, it relies on the order of response arrival to correspond exactly to the order in which requests are made on the same connection. More than one response message per request only occurs when one or more informational responses (1xx, see Section 6.2 of [Part2]) precede a final response to the same request.¶

A client that has more than one outstanding request on a connection MUST maintain a list of outstanding requests in the order sent and MUST associate each received response message on that connection to the highest ordered request that has not yet received a final (non-1xx) response.¶

As described in Section 2.3, intermediaries can serve a variety of roles in the processing of HTTP requests and responses. Some intermediaries are used to improve performance or availability. Others are used for access control or to filter content. Since an HTTP stream has characteristics similar to a pipe-and-filter architecture, there are no inherent limits to the extent an intermediary can enhance (or interfere) with either direction of the stream.¶

Intermediaries that forward a message MUST implement the Connection header field, as specified in Section 6.1, to exclude fields that are only intended for the incoming connection.¶

In order to avoid request loops, a proxy that forwards requests to other proxies MUST be able to recognize and exclude all of its own server names, including any aliases, local variations, or literal IP addresses.¶

The "Connection" header field allows the sender to indicate desired control options for the current connection. In order to avoid confusing downstream recipients, a proxy or gateway MUST remove or replace any received connection options before forwarding the message.¶

When a header field aside from Connection is used to supply control information for or about the current connection, the sender MUST list the corresponding field-name within the "Connection" header field. A proxy or gateway MUST parse a received Connection header field before a message is forwarded and, for each connection-option in this field, remove any header field(s) from the message with the same name as the connection-option, and then remove the Connection header field itself (or replace it with the intermediary's own connection options for the forwarded message).¶

Hence, the Connection header field provides a declarative way of distinguishing header fields that are only intended for the immediate recipient ("hop-by-hop") from those fields that are intended for all recipients on the chain ("end-to-end"), enabling the message to be self-descriptive and allowing future connection-specific extensions to be deployed without fear that they will be blindly forwarded by older intermediaries.¶

Connection options are case-insensitive.¶

A sender MUST NOT send a connection option corresponding to a header field that is intended for all recipients of the payload. For example, Cache-Control is never appropriate as a connection option (Section 7.2 of [Part6]).¶

The connection options do not have to correspond to a header field present in the message, since a connection-specific header field might not be needed if there are no parameters associated with that connection option. Recipients that trigger certain connection behavior based on the presence of connection options MUST do so based on the presence of the connection-option rather than only the presence of the optional header field. In other words, if the connection option is received as a header field but not indicated within the Connection field-value, then the recipient MUST ignore the connection-specific header field because it has likely been forwarded by an intermediary that is only partially conformant.¶

When defining new connection options, specifications ought to carefully consider existing deployed header fields and ensure that the new connection option does not share the same name as an unrelated header field that might already be deployed. Defining a new connection option essentially reserves that potential field-name for carrying additional information related to the connection option, since it would be unwise for senders to use that field-name for anything else.¶

The "close" connection option is defined for a sender to signal that this connection will be closed after completion of the response. For example,¶

in either the request or the response header fields indicates that the connection MUST be closed after the current request/response is complete (Section 6.6).¶

A client that does not support persistent connections MUST send the "close" connection option in every request message.¶

A server that does not support persistent connections MUST send the "close" connection option in every response message that does not have a 1xx (Informational) status code.¶

It is beyond the scope of this specification to describe how connections are established via various transport or session-layer protocols. Each connection applies to only one transport link.¶

HTTP/1.1 defaults to the use of "persistent connections", allowing multiple requests and responses to be carried over a single connection. The "close" connection-option is used to signal that a connection will not persist after the current request/response. HTTP implementations SHOULD support persistent connections.¶

A recipient determines whether a connection is persistent or not based on the most recently received message's protocol version and Connection header field (if any): ¶

• If the close connection option is present, the connection will not persist after the current response; else,
• If the received protocol is HTTP/1.1 (or later), the connection will persist after the current response; else,
• If the received protocol is HTTP/1.0, the "keep-alive" connection option is present, the recipient is not a proxy, and the recipient wishes to honor the HTTP/1.0 "keep-alive" mechanism, the connection will persist after the current response; otherwise,
• The connection will close after the current response.

A server MAY assume that an HTTP/1.1 client intends to maintain a persistent connection until a close connection option is received in a request.¶

A client MAY reuse a persistent connection until it sends or receives a close connection option or receives an HTTP/1.0 response without a "keep-alive" connection option.¶

In order to remain persistent, all messages on a connection MUST have a self-defined message length (i.e., one not defined by closure of the connection), as described in Section 3.3. A server MUST read the entire request message body or close the connection after sending its response, since otherwise the remaining data on a persistent connection would be misinterpreted as the next request. Likewise, a client MUST read the entire response message body if it intends to reuse the same connection for a subsequent request.¶

A proxy server MUST NOT maintain a persistent connection with an HTTP/1.0 client (see Section 19.7.1 of [RFC2068] for information and discussion of the problems with the Keep-Alive header field implemented by many HTTP/1.0 clients).¶

Clients and servers SHOULD NOT assume that a persistent connection is maintained for HTTP versions less than 1.1 unless it is explicitly signaled. See Appendix A.1.2 for more information on backward compatibility with HTTP/1.0 clients.¶

Clients SHOULD limit the number of simultaneous connections that they maintain to a given server.¶

Previous revisions of HTTP gave a specific number of connections as a ceiling, but this was found to be impractical for many applications. As a result, this specification does not mandate a particular maximum number of connections, but instead encourages clients to be conservative when opening multiple connections.¶

Multiple connections are typically used to avoid the "head-of-line blocking" problem, wherein a request that takes significant server-side processing and/or has a large payload blocks subsequent requests on the same connection. However, each connection consumes server resources. Furthermore, using multiple connections can cause undesirable side effects in congested networks.¶

Note that servers might reject traffic that they deem abusive, including an excessive number of connections from a client.¶

Servers will usually have some time-out value beyond which they will no longer maintain an inactive connection. Proxy servers might make this a higher value since it is likely that the client will be making more connections through the same server. The use of persistent connections places no requirements on the length (or existence) of this time-out for either the client or the server.¶

When a client or server wishes to time-out it SHOULD issue a graceful close on the transport connection. Clients and servers SHOULD both constantly watch for the other side of the transport close, and respond to it as appropriate. If a client or server does not detect the other side's close promptly it could cause unnecessary resource drain on the network.¶

A client, server, or proxy MAY close the transport connection at any time. For example, a client might have started to send a new request at the same time that the server has decided to close the "idle" connection. From the server's point of view, the connection is being closed while it was idle, but from the client's point of view, a request is in progress.¶

Servers SHOULD maintain persistent connections and allow the underlying transport's flow control mechanisms to resolve temporary overloads, rather than terminate connections with the expectation that clients will retry. The latter technique can exacerbate network congestion.¶

A client sending a message body SHOULD monitor the network connection for an error status code while it is transmitting the request. If the client sees an error status code, it SHOULD immediately cease transmitting the body and close the connection.¶

The Connection header field (Section 6.1) provides a "close" connection option that a sender SHOULD send when it wishes to close the connection after the current request/response pair.¶

A client that sends a close connection option MUST NOT send further requests on that connection (after the one containing close) and MUST close the connection after reading the final response message corresponding to this request.¶

A server that receives a close connection option MUST initiate a lingering close (see below) of the connection after it sends the final response to the request that contained close. The server SHOULD send a close connection option in its final response on that connection. The server MUST NOT process any further requests received on that connection.¶

A server that sends a close connection option MUST initiate a lingering close of the connection after it sends the response containing close. The server MUST NOT process any further requests received on that connection.¶

A client that receives a close connection option MUST cease sending requests on that connection and close the connection after reading the response message containing the close; if additional pipelined requests had been sent on the connection, the client SHOULD assume that they will not be processed by the server.¶

If a server performs an immediate close of a TCP connection, there is a significant risk that the client will not be able to read the last HTTP response. If the server receives additional data from the client on a fully-closed connection, such as another request that was sent by the client before receiving the server's response, the server's TCP stack will send a reset packet to the client; unfortunately, the reset packet might erase the client's unacknowledged input buffers before they can be read and interpreted by the client's HTTP parser.¶

To avoid the TCP reset problem, a server can perform a lingering close on a connection by closing only the write side of the read/write connection (a half-close) and continuing to read from the connection until the connection is closed by the client or the server is reasonably certain that its own TCP stack has received the client's acknowledgement of the packet(s) containing the server's last response. It is then safe for the server to fully close the connection.¶

It is unknown whether the reset problem is exclusive to TCP or might also be found in other transport connection protocols.¶

The "Upgrade" header field is intended to provide a simple mechanism for transitioning from HTTP/1.1 to some other protocol on the same connection. A client MAY send a list of protocols in the Upgrade header field of a request to invite the server to switch to one or more of those protocols before sending the final response. A server MUST send an Upgrade header field in 101 (Switching Protocols) responses to indicate which protocol(s) are being switched to, and MUST send it in 426 (Upgrade Required) responses to indicate acceptable protocols. A server MAY send an Upgrade header field in any other response to indicate that they might be willing to upgrade to one of the specified protocols for a future request.¶

For example,¶

Upgrade eases the difficult transition between incompatible protocols by allowing the client to initiate a request in the more commonly supported protocol while indicating to the server that it would like to use a "better" protocol if available (where "better" is determined by the server, possibly according to the nature of the request method or target resource).¶

Upgrade cannot be used to insist on a protocol change; its acceptance and use by the server is optional. The capabilities and nature of the application-level communication after the protocol change is entirely dependent upon the new protocol chosen, although the first action after changing the protocol MUST be a response to the initial HTTP request that contained the Upgrade header field.¶

For example, if the Upgrade header field is received in a GET request and the server decides to switch protocols, then it first responds with a 101 (Switching Protocols) message in HTTP/1.1 and then immediately follows that with the new protocol's equivalent of a response to a GET on the target resource. This allows a connection to be upgraded to protocols with the same semantics as HTTP without the latency cost of an additional round-trip. A server MUST NOT switch protocols unless the received message semantics can be honored by the new protocol; an OPTIONS request can be honored by any protocol.¶

When Upgrade is sent, a sender MUST also send a Connection header field (Section 6.1) that contains the "upgrade" connection option, in order to prevent Upgrade from being accidentally forwarded by intermediaries that might not implement the listed protocols. A server MUST ignore an Upgrade header field that is received in an HTTP/1.0 request.¶

The Upgrade header field only applies to switching application-level protocols on the existing connection; it cannot be used to switch to a protocol on a different connection. For that purpose, it is more appropriate to use a 3xx (Redirection) response (Section 6.4 of [Part2]).¶

This specification only defines the protocol name "HTTP" for use by the family of Hypertext Transfer Protocols, as defined by the HTTP version rules of Section 2.6 and future updates to this specification. Additional tokens ought to be registered with IANA using the registration procedure defined in Section 7.6.¶

HTTP header fields are registered within the Message Header Field Registry [BCP90] maintained by IANA at <http://www.iana.org/assignments/message-headers/message-header-index.html>.¶

Furthermore, the header field-name "Close" shall be registered as "reserved", since using that name as an HTTP header field might conflict with the "close" connection option of the "Connection" header field (Section 6.1).¶

The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".¶

IANA maintains the registry of URI Schemes [BCP115] at <http://www.iana.org/assignments/uri-schemes.html>.¶

This document serves as the specification for the Internet media types "message/http" and "application/http". The following is to be registered with IANA (see [BCP13]).¶

The HTTP Transfer Coding Registry defines the name space for transfer coding names.¶

Registrations MUST include the following fields: ¶

• Name
• Description
• Pointer to specification text

Names of transfer codings MUST NOT overlap with names of content codings (Section 3.1.2.1 of [Part2]) unless the encoding transformation is identical, as is the case for the compression codings defined in Section 4.2.¶

Values to be added to this name space require IETF Review (see Section 4.1 of [RFC5226]), and MUST conform to the purpose of transfer coding defined in this section. Use of program names for the identification of encoding formats is not desirable and is discouraged for future encodings.¶

The registry itself is maintained at <http://www.iana.org/assignments/http-parameters>.¶

The HTTP Upgrade Token Registry defines the name space for protocol-name tokens used to identify protocols in the Upgrade header field. Each registered protocol name is associated with contact information and an optional set of specifications that details how the connection will be processed after it has been upgraded.¶

Registrations happen on a "First Come First Served" basis (see Section 4.1 of [RFC5226]) and are subject to the following rules: ¶

1. A protocol-name token, once registered, stays registered forever.
2. The registration MUST name a responsible party for the registration.
3. The registration MUST name a point of contact.
4. The registration MAY name a set of specifications associated with that token. Such specifications need not be publicly available.
5. The registration SHOULD name a set of expected "protocol-version" tokens associated with that token at the time of registration.
6. The responsible party MAY change the registration at any time. The IANA will keep a record of all such changes, and make them available upon request.
7. The IESG MAY reassign responsibility for a protocol token. This will normally only be used in the case when a responsible party cannot be contacted.

This registration procedure for HTTP Upgrade Tokens replaces that previously defined in Section 7.2 of [RFC2817].¶

The responsible party is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".¶

HTTP clients rely heavily on the Domain Name Service (DNS), and are thus generally prone to security attacks based on the deliberate misassociation of IP addresses and DNS names not protected by DNSSEC. Clients need to be cautious in assuming the validity of an IP number/DNS name association unless the response is protected by DNSSEC ([RFC4033]).¶

By their very nature, HTTP intermediaries are men-in-the-middle, and represent an opportunity for man-in-the-middle attacks. Compromise of the systems on which the intermediaries run can result in serious security and privacy problems. Intermediaries have access to security-related information, personal information about individual users and organizations, and proprietary information belonging to users and content providers. A compromised intermediary, or an intermediary implemented or configured without regard to security and privacy considerations, might be used in the commission of a wide range of potential attacks.¶

Intermediaries that contain a shared cache are especially vulnerable to cache poisoning attacks.¶

Implementers need to consider the privacy and security implications of their design and coding decisions, and of the configuration options they provide to operators (especially the default configuration).¶

Users need to be aware that intermediaries are no more trustworthy than the people who run them; HTTP itself cannot solve this problem.¶

Because HTTP uses mostly textual, character-delimited fields, attackers can overflow buffers in implementations, and/or perform a Denial of Service against implementations that accept fields with unlimited lengths.¶

To promote interoperability, this specification makes specific recommendations for minimum size limits on request-line (Section 3.1.1) and blocks of header fields (Section 3.2). These are minimum recommendations, chosen to be supportable even by implementations with limited resources; it is expected that most implementations will choose substantially higher limits.¶

This specification also provides a way for servers to reject messages that have request-targets that are too long (Section 6.5.12 of [Part2]) or request entities that are too large (Section 6.5 of [Part2]).¶

Recipients SHOULD carefully limit the extent to which they read other fields, including (but not limited to) request methods, response status phrases, header field-names, and body chunks, so as to avoid denial of service attacks without impeding interoperability.¶

HTTP does not define a specific mechanism for ensuring message integrity, instead relying on the error-detection ability of underlying transport protocols and the use of length or chunk-delimited framing to detect completeness. Additional integrity mechanisms, such as hash functions or digital signatures applied to the content, can be selectively added to messages via extensible metadata header fields. Historically, the lack of a single integrity mechanism has been justified by the informal nature of most HTTP communication. However, the prevalence of HTTP as an information access mechanism has resulted in its increasing use within environments where verification of message integrity is crucial.¶

User agents are encouraged to implement configurable means for detecting and reporting failures of message integrity such that those means can be enabled within environments for which integrity is necessary. For example, a browser being used to view medical history or drug interaction information needs to indicate to the user when such information is detected by the protocol to be incomplete, expired, or corrupted during transfer. Such mechanisms might be selectively enabled via user agent extensions or the presence of message integrity metadata in a response. At a minimum, user agents ought to provide some indication that allows a user to distinguish between a complete and incomplete response message (Section 3.4) when such verification is desired.¶

A server is in the position to save personal data about a user's requests over time, which might identify their reading patterns or subjects of interest. In particular, log information gathered at an intermediary often contains a history of user agent interaction, across a multitude of sites, that can be traced to individual users.¶

HTTP log information is confidential in nature; its handling is often constrained by laws and regulations. Log information needs to be securely stored and appropriate guidelines followed for its analysis. Anonymization of personal information within individual entries helps, but is generally not sufficient to prevent real log traces from being re-identified based on correlation with other access characteristics. As such, access traces that are keyed to a specific client should not be published even if the key is pseudonymous.¶

To minimize the risk of theft or accidental publication, log information should be purged of personally identifiable information, including user identifiers, IP addresses, and user-provided query parameters, as soon as that information is no longer necessary to support operational needs for security, auditing, or fraud control.¶

10.1. Normative References

[Part2]

Fielding, R., Ed. and J. Reschke, Ed., “Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content”, Internet-Draft draft-ietf-httpbis-p2-semantics-22 (work in progress), February 2013.

[Part4]

Fielding, R., Ed. and J. Reschke, Ed., “Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests”, Internet-Draft draft-ietf-httpbis-p4-conditional-22 (work in progress), February 2013.

[Part5]

Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed., “Hypertext Transfer Protocol (HTTP/1.1): Range Requests”, Internet-Draft draft-ietf-httpbis-p5-range-22 (work in progress), February 2013.

[Part6]

Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., “Hypertext Transfer Protocol (HTTP/1.1): Caching”, Internet-Draft draft-ietf-httpbis-p6-cache-22 (work in progress), February 2013.

[Part7]

Fielding, R., Ed. and J. Reschke, Ed., “Hypertext Transfer Protocol (HTTP/1.1): Authentication”, Internet-Draft draft-ietf-httpbis-p7-auth-22 (work in progress), February 2013.

[RFC1950]

Deutsch, L. and J-L. Gailly, “ZLIB Compressed Data Format Specification version 3.3”, RFC 1950, May 1996.

[RFC1951]

Deutsch, P., “DEFLATE Compressed Data Format Specification version 1.3”, RFC 1951, May 1996.

[RFC1952]

Deutsch, P., Gailly, J-L., Adler, M., Deutsch, L., and G. Randers-Pehrson, “GZIP file format specification version 4.3”, RFC 1952, May 1996.

[RFC2119]

Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels”, BCP 14, RFC 2119, March 1997.

[RFC3986]

Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax”, STD 66, RFC 3986, January 2005.

[RFC5234]

Crocker, D., Ed. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF”, STD 68, RFC 5234, January 2008.

[USASCII]

American National Standards Institute, “Coded Character Set -- 7-bit American Standard Code for Information Interchange”, ANSI X3.4, 1986.

10.2. Informative References

[BCP115]

Hansen, T., Hardie, T., and L. Masinter, “Guidelines and Registration Procedures for New URI Schemes”, BCP 115, RFC 4395, February 2006.

[BCP13]

Freed, N., Klensin, J., and T. Hansen, “Media Type Specifications and Registration Procedures”, BCP 13, RFC 6838, January 2013.

[BCP90]

Klyne, G., Nottingham, M., and J. Mogul, “Registration Procedures for Message Header Fields”, BCP 90, RFC 3864, September 2004.

[ISO-8859-1]

International Organization for Standardization, “Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1”, ISO/IEC 8859-1:1998, 1998.

[Kri2001]

Kristol, D., “HTTP Cookies: Standards, Privacy, and Politics”, ACM Transactions on Internet Technology Vol. 1, #2, November 2001, <http://arxiv.org/abs/cs.SE/0105018>.

[RFC1919]

Chatel, M., “Classical versus Transparent IP Proxies”, RFC 1919, March 1996.

[RFC1945]

Berners-Lee, T., Fielding, R., and H. Nielsen, “Hypertext Transfer Protocol -- HTTP/1.0”, RFC 1945, May 1996.

[RFC2045]

Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies”, RFC 2045, November 1996.

[RFC2047]

Moore, K., “MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text”, RFC 2047, November 1996.

[RFC2068]

Fielding, R., Gettys, J., Mogul, J., Nielsen, H., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1”, RFC 2068, January 1997.

[RFC2145]

Mogul, J., Fielding, R., Gettys, J., and H. Nielsen, “Use and Interpretation of HTTP Version Numbers”, RFC 2145, May 1997.

[RFC2616]

Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1”, RFC 2616, June 1999.

[RFC2817]

Khare, R. and S. Lawrence, “Upgrading to TLS Within HTTP/1.1”, RFC 2817, May 2000.

[RFC2818]

Rescorla, E., “HTTP Over TLS”, RFC 2818, May 2000.

[RFC3040]

Cooper, I., Melve, I., and G. Tomlinson, “Internet Web Replication and Caching Taxonomy”, RFC 3040, January 2001.

[RFC4033]

Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “DNS Security Introduction and Requirements”, RFC 4033, March 2005.

[RFC4559]

Jaganathan, K., Zhu, L., and J. Brezak, “SPNEGO-based Kerberos and NTLM HTTP Authentication in Microsoft Windows”, RFC 4559, June 2006.

[RFC5226]

Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs”, BCP 26, RFC 5226, May 2008.

[RFC5246]

Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2”, RFC 5246, August 2008.

[RFC5322]

Resnick, P., “Internet Message Format”, RFC 5322, October 2008.

[RFC6265]

Barth, A., “HTTP State Management Mechanism”, RFC 6265, April 2011.

This section summarizes major differences between versions HTTP/1.0 and HTTP/1.1.¶

HTTP's approach to error handling has been explained. (Section 2.5)¶

The expectation to support HTTP/0.9 requests has been removed.¶

The term "Effective Request URI" has been introduced. (Section 5.5)¶

HTTP messages can be (and often are) buffered by implementations; despite it sometimes being available as a stream, HTTP is fundamentally a message-oriented protocol. (Section 3)¶

Minimum supported sizes for various protocol elements have been suggested, to improve interoperability.¶

Header fields that span multiple lines ("line folding") are deprecated. (Section 3.2.4)¶

The HTTP-version ABNF production has been clarified to be case-sensitive. Additionally, version numbers has been restricted to single digits, due to the fact that implementations are known to handle multi-digit version numbers incorrectly. (Section 2.6)¶

The HTTPS URI scheme is now defined by this specification; previously, it was done in Section 2.4 of [RFC2818]. (Section 2.7.2)¶

The HTTPS URI scheme implies end-to-end security. (Section 2.7.2)¶

Userinfo (i.e., username and password) are now disallowed in HTTP and HTTPS URIs, because of security issues related to their transmission on the wire. (Section 2.7.1)¶

Invalid whitespace around field-names is now required to be rejected, because accepting it represents a security vulnerability. (Section 3.2)¶

The ABNF productions defining header fields now only list the field value. (Section 3.2)¶

Rules about implicit linear whitespace between certain grammar productions have been removed; now whitespace is only allowed where specifically defined in the ABNF. (Section 3.2.3)¶

The NUL octet is no longer allowed in comment and quoted-string text, and handling of backslash-escaping in them has been clarified. (Section 3.2.6)¶

The quoted-pair rule no longer allows escaping control characters other than HTAB. (Section 3.2.6)¶

Non-ASCII content in header fields and the reason phrase has been obsoleted and made opaque (the TEXT rule was removed). (Section 3.2.6)¶

Bogus "Content-Length" header fields are now required to be handled as errors by recipients. (Section 3.3.2)¶

The "identity" transfer coding token has been removed. (Sections 3.3 and 4)¶

The algorithm for determining the message body length has been clarified to indicate all of the special cases (e.g., driven by methods or status codes) that affect it, and that new protocol elements cannot define such special cases. (Section 3.3.3)¶

"multipart/byteranges" is no longer a way of determining message body length detection. (Section 3.3.3)¶

CONNECT is a new, special case in determining message body length. (Section 3.3.3)¶

Chunk length does not include the count of the octets in the chunk header and trailer. (Section 4.1)¶

Use of chunk extensions is deprecated, and line folding in them is disallowed. (Section 4.1)¶

The segment + query components of RFC3986 have been used to define the request-target, instead of abs_path from RFC 1808. (Section 5.3)¶

The asterisk form of the request-target is only allowed in the OPTIONS method. (Section 5.3)¶

Exactly when "close" connection options have to be sent has been clarified. (Section 6.1)¶

"hop-by-hop" header fields are required to appear in the Connection header field; just because they're defined as hop-by-hop in this specification doesn't exempt them. (Section 6.1)¶

The limit of two connections per server has been removed. (Section 6.3)¶

An idempotent sequence of requests is no longer required to be retried. (Section 6.3)¶

The requirement to retry requests under certain circumstances when the server prematurely closes the connection has been removed. (Section 6.3)¶

Some extraneous requirements about when servers are allowed to close connections prematurely have been removed. (Section 6.3)¶

The semantics of the Upgrade header field is now defined in responses other than 101 (this was incorporated from [RFC2817]). (Section 6.7)¶

Registration of Transfer Codings now requires IETF Review (Section 7.4)¶

The meaning of the "deflate" content coding has been clarified. (Section 4.2.2)¶

This specification now defines the Upgrade Token Registry, previously defined in Section 7.2 of [RFC2817]. (Section 7.6)¶

Empty list elements in list productions (e.g., a list header containing ", ,") have been deprecated. (Appendix B)¶

Issues with the Keep-Alive and Proxy-Connection headers in requests are pointed out, with use of the latter being discouraged altogether. (Appendix A.1.2)¶

Changes up to the first Working Group Last Call draft are summarized in <http://trac.tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-21#appendix-D>.¶

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/128>: "Cite HTTPS URI scheme definition" (the spec now includes the HTTPs scheme definition and thus updates RFC 2818)
• <http://tools.ietf.org/wg/httpbis/trac/ticket/389>: "mention of 'proxies' in section about caches"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/390>: "use of ABNF terms from RFC 3986"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/392>: "editorial improvements to message length definition"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/395>: "Connection header field MUST vs SHOULD"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/396>: "editorial improvements to persistent connections section"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/397>: "URI normalization vs empty path"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/408>: "p1 feedback"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/409>: "is parsing OBS-FOLD mandatory?"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/410>: "HTTPS and Shared Caching"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/411>: "Requirements for recipients of ws between start-line and first header field"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/412>: "SP and HT when being tolerant"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/414>: "Message Parsing Strictness"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/415>: "'Render'"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/418>: "No-Transform"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/419>: "p2 editorial feedback"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/420>: "Content-Length SHOULD be sent"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/431>: "origin-form does not allow path starting with "//""
• <http://tools.ietf.org/wg/httpbis/trac/ticket/433>: "ambiguity in part 1 example"